Photocatalyst carrier and manufacturing method therefor

ABSTRACT

A photocatalyst carrier is described, characterized in that the surface and/or pores of a hot-melt resin porous article is/are coated with a photocatalytic substance, and the system is heated to the softening temperature of the hot-melt resin to allow the photocatalytic substance to penetrate to some extent into the surface and/or pores of the hot-melt resin porous article, providing sufficient exposure to the surrounding atmosphere for the surface of the photocatalytic substance and supporting the substance on the hot-melt resin porous article with adequate bonding force.

BACKGROUND OF THE INVENTION

The present invention relates to a photocatalyst carrier for supportinga substance having a photocatalytic function (for example, titaniumdioxide), and manufacturing method therefor.

DESCRIPTION OF RELATED ART

Conventionally known photocatalyst carriers include (1) those obtainedby methods in which a porous membrane is impregnated with a stocksolution of a photocatalytic particulate substance, and this stocksolution is then subjected to a pulverization reaction; (2) thoseobtained by methods in which voids in a porous membrane are impregnatedwith a dispersion of a photocatalytic particulate substance, and theproduct is dried and supported; and (3) those obtained by methods inwhich a photocatalytic particulate substance is mixed, for example, withpolytetrafluoroethylene (hereinafter “PTFE”), and is then made into awoven article after being calendered into a sheet or porosified andfibrillated.

Carriers obtained by methods in which a porous membrane is merelyimpregnated with a stock solution of a photocatalytic particulatesubstance, which is then subjected to a pulverization reaction, orcarriers obtained by methods in which a photocatalytic particulatesubstance is directly supported by impregnation are disadvantageous,however, in that the photocatalytic substance and the porous membraneare not bonded together particularly strongly, and the photocatalyticsubstance tends to peel off. To address this problem, a method in whicha photocatalytic particulate substance is tacked on to the surface of aPTFE porous membrane or the like, and this particulate substance is thencompression-bonded with the PTFE porous membrane by being pressed withrolls is disclosed, for example, in Japanese Patent Application(Tokugan) 1-101342. In such a method, however, fixing is achieved bymechanical pressure alone, making it impossible to achieve sufficientbonding between the photocatalytic particulate substance and the PTFEporous membrane.

In addition, porous products made by premixing a photocatalyticparticulate substance with a substrate resin such as PTFE develop strongbonding force between the photocatalytic particulate substance and thePTFE resin itself, but the surface of the photocatalytic particulatesubstance is not exposed adequately, and the resulting efficiency of thephotocatalytic function is low. Furthermore, admixing large amounts ofphotocatalytic substances in order to raise the efficiency of thephotocatalytic function tends to cause problems associated with a markedreduction in the mechanical strength of the PTFE porous article.

An object of the present invention is to provide a highly durablephotocatalyst carrier in which sufficient exposure to the surroundingatmosphere is achieved for the surface of the photocatalytic substanceon the surface and/or in the pores of a hot-melt resin porous article,optionally provided with a reticulated layer, and in which thephotocatalytic substance is firmly supported by the photocatalyticsubstance.

SUMMARY OF THE INVENTION

According to the present invention, a substance having a photocatalyticfunction (such as titanium dioxide particles) is applied to a hot-meltresin porous article, and the article is then heated to the softeningtemperature of the resin surface, whereupon the pores of the hot-meltresin porous article shrink somewhat, and the surface softens at thesame time, with the result that the applied titanium dioxide particlesare supported by, and firmly bonded to, the hot-melt resin porousarticle while somewhat embedded in the pore walls.

Compared with cases in which the resin surface is merely coated withdeposited titanium dioxide, therefore, the photocatalyst carrierpertaining to the present invention can develop a stronger bondingforce, and the titanium dioxide can maintain its catalytic function fora long time with virtually no peeling or separation from the hot meltresin porous article, even under fairly rigorous service conditions.

In addition, when the hot-melt resin porous article that supports theaforementioned titanium dioxide is used in the form of a flat membrane,it is possible to fabricate a reticulated product with high mechanicalstrength, such as a photocatalyst-carrying sheet that is resistant tostretching or deformation and that is obtained by integrating a layer ofglass fabric with the aid of a laminator. In the photocatalyst-carryingsheet thus fabricated, the photocatalytic substance remains resistant topeeling when stretched or the like. Another benefit is that in contrastto the mixing and molding of titanium dioxide and resins, large areas onthe surface of the titanium dioxide are not penetrated into the resin,and thus, are exposed on the surface, making it possible to increase thesurface area of the titanium dioxide functioning as a photocatalyst.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described in detail through specificembodiments.

A hot-melt resin porous article is used as the substrate for thephotocatalyst carrier of the present invention. Using a fluororesin asthe hot-melt resin is particularly preferred because of the need toafford oxidation resistance, taking into account the decomposition powerof the active oxygen produced by titanium dioxide. Specific examples ofresins that can be used as such hot-melt fluororesins includetetrafluoroethylene-hexafluoroethylene copolymers (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),tetrafluoroethylene-hexafluoroethylene-perfluoroalkyl vinyl ethercopolymers (EPA), and tetrafluoroethylene-ethylene copolymers (ETFE). Inaddition, the aforementioned hot-melt fluororesins, and PFA and FEP inparticular, exhibit good transmission properties at a wavelength of 385nm, which corresponds to the light (UV light) needed to achievephotocatalysis, and are thus suitable as such photocatalyst carriersubstrates. Another feature of such hot-melt fluororesins is that theirmelt viscosity falls within a range of about 102 to 107 poise; that is,these resins behave similarly to common macromolecular materials and,unlike low-molecular substances, melt within a fairly wide temperaturerange, with the result that these resins can be adequately softened byheating without causing the collapse of the pores in the porous article.

A porous article used as a substrate can be obtained from theaforementioned fluororesin by a method in which hot-melt fluororesinparticles and a thermoplastic resin soluble in organic solvents areheated to a temperature above the melting point of the thermoplasticresin soluble in organic solvents but below the melting point of thehot-melt fluororesin. Pressure and shearing force are optionally appliedat the same time as heat is applied to perform molding. The system issubsequently heat-treated at a temperature above the melting point ofthe hot-melt fluororesin, and the resin soluble in organic solvent isthen dissolved away with an organic solvent, yielding a hot-meltfluororesin porous article.

It is also possible to obtain a hot-melt fluororesin porous sheetreinforced with glass fabric by performing the following operationsduring the manufacturing steps. A sheet is first molded when the systemis heated and molded at a temperature below the melting point of thehot-melt fluororesin, as described in the molding steps above. Glassfabric is then sandwiched and integrated between two such sheets, thenthis system is heat-treated at a temperature above the melting point ofthe hot-melt fluororesin, and the resin soluble in organic solvents isthen dissolved away with an organic solvent. The pores and/or thesurface of the porous article are/is coated with titanium dioxide by amethod in which the resulting hot-melt fluororesin porous article,optionally reinforced with glass fabric, is impregnated with a solutionobtained by dispersing fine titanium dioxide particles in water, and thesystem is then dried, or by a method in which a titanium compoundserving as a starting material for fine titanium dioxide particles iscaused to react directly with the aforementioned hot-melt fluororesinporous article by a sol-gel technique. The system is then heated to atemperature 30-80° C. below the melting point of the hot-meltfluororesin. The titanium dioxide deposited on the hot-melt fluororesinporous article is thereby held on the porous article by a strong bondingforce. This is attributed to the fact that the surfaces and pore wallsof the hot-melt fluororesin porous article are softened, and the entirepores are somewhat contracted to enclose the titanium dioxide such thatthe titanium dioxide that covers the surfaces and pores of the resinpenetrates into the resin to a certain extent, demonstrating ananchoring effect and producing considerable bonding force. Thecorresponding temperature should therefore be set to the softeningtemperature of the resin surface. An excessively low temperature willmake it impossible to support titanium dioxide on the resin withadequate bonding force, whereas an excessively high temperature willresult in complete melting and will cause the pores to collapse. Thetemperature must therefore be selected such that surface softening isinduced in the hot-melt fluororesin used.

As a result of being subjected to such a process, the titanium dioxideis supported with substantial bonding force on the hot-melt fluororesinporous article. In addition, the titanium dioxide has a wide exposuresurface while penetrating only slightly into the pores or near thesurface of the resin, so the surface area of the titanium dioxide havingcatalytic action can be utilized with high efficiency.

When the hot-melt fluororesin porous article is required to haveparticularly pronounced hydrophilic properties, it is possible toperform a hydrophilization treatment to provide such hydrophilicproperties. In one preferred method, the aforementioned hot-meltfluororesin porous article, before being coated with titanium dioxide,is impregnated with a dispersion of fine particulate silicon dioxide oranother inorganic substance having pronounced hydrophilic properties,then dried and heat-treated. In another method, a stock solution of asilicon dioxide powder substance is pulverized, and the resulting powderis impregnated into the article and bonded by a sol-gel technique tocover the aforementioned hot-melt fluororesin porous article with thesilicon dioxide. In the particular case of ETFE being used as thesubstrate, applying silicon dioxide is effective not only for achievingbetter hydrophilic properties but also for preventing the ETFE frombeing corroded as a result of the strong photocatalytic action oftitanium dioxide. The reason is that the ETFE is less likely to bedirectly affected by the titanium dioxide because of the presence of asilicon dioxide layer between the titanium dioxide and ETFE.

Apart from titanium dioxide, other examples of photocatalytic substancesinclude zinc oxide, iron oxide, cadmium sulfide, cadmium selenide, andstrontium titanate. These photocatalysts may be used singly or ascombinations of two or more photocatalysts.

Aluminum oxide and zirconium oxide can be cited in addition to silicondioxide as examples of the inorganic oxides used for thehydrophilization of the hot-melt fluororesin porous article, but anysubstance may be used as long as it has a hydrophilization function, isnot affected by photocatalysis, and performs functions similar to thoseof the aforementioned silicon dioxide.

The photocatalyst carrier can be shaped as a flat membrane or a hollowyarn, and is not subject to any particular limitations as long it has ashape that can be assumed by the hot-melt fluororesin porous articleused as the substrate.

In addition, the reticulated material used as reinforcement in thehot-melt fluororesin porous article is not limited to glass fabric andcan be any material that is impervious to the photocatalytic action oftitanium dioxide, has excellent mechanical strength, and can belaminated to the hot-melt fluororesin porous article. The location inwhich this material is provided can also be varied freely depending onthe intended application.

Without intending to limit the scope of the present invention, thefollowing examples illustrate how the present invention may be made andused:

EXAMPLE 1

The following two components were mixed at room temperature: 1.0 kgaqueous PFA dispersion (Dainion, PFA 6910N, solids: 23 wt %) and 8.2 kgof a 10-wt % acetone solution of a tetrafluoroethylene-vinylidenefluoride copolymer (Daikin Industries, Neoflon VDF, hereinafter “VDF”).The mixture was gelated, and the solids were taken out and dried. Thesolids were subsequently pelletized in a twin-screw extruder at a dietemperature of 160° C. The resulting pellets were melt-extruded at a dietemperature of 200° C. with the aid of a single-screw extruder equippedwith a slit die at the tip, yielding a sheet with a thickness of about0.14 mm.

The sheet was then calendered to about 70% of its original thickness bybeing passed through a roll press heated to about 160° C. As a result ofthese operations, the PFA particles in the ultimately obtained PFAporous article were somewhat fibrillated and made into athree-dimensional bonded reticulated structure. The sheet wassubsequently kept for about 20 seconds in an electric oven at 320° C.and then dipped in acetone to dissolve away the VDF. The porous PFAsheet thus obtained had the above-described three-dimensionalreticulated structure and possessed a porosity of 80% and a thickness of0.1 mm.

A titanium dioxide dispersion (Tinoc CZG-221, manufactured by TakiChemical, titanium dioxide with average particle size of severalnanometers) was subsequently added in an amount of 400 g to a solutionobtained by mixing 160 g water, 240 g isopropanol, and 5 g of a 1-wt %aqueous solution of a surfactant (FC-170C, manufactured by 3M), yieldingTreatment Solution 1. The pores in the sheet were impregnated with thetitanium dioxide dispersion either by immersing the porous PFA sheet inTreatment Solution 1 or spraying Treatment Solution 1 over the porousPFA sheet with a sprayer. The porous PFA sheet was subsequently driedwith hot air (100° C. or less), washed with water, and re-dried. Theresulting porous PFA sheet that supported titanium dioxide was kept at atemperature of 270-290° C. in an electric oven.

A porous PFA sheet that supported titanium dioxide was obtained in themanner described above, a product obtained by coating drawn porous PTFEwith titanium dioxide by a sol-gel technique was used as a comparisonsample, and the two products were tested by comparing the bonding forceswith which the titanium dioxide was supported. In these tests,self-adhesive tape (cellophane tape) was pasted over the same surfacearea of the two products, and this self-adhesive tape was then peeledoff to determine the degree to which titanium dioxide had adhered to thesurfaces of the self-adhesive layers. As a result, no titanium dioxidehad adhered to the surface of the self-adhesive layer on theself-adhesive tape peeled off from the porous PFA sheet that supportedthe titanium dioxide in Example 1 of the present invention. In contrastthe entire surface of the self-adhesive layer was covered with titaniumdioxide in the case of the self-adhesive layer on the self-adhesive tapepeeled off from the product obtained in accordance with the comparativeexample by supporting titanium dioxide on stretched porous PTFE.

EXAMPLE 2

The following two components were mixed at room temperature: 1.6 kgaqueous PFA dispersion (Dainion, PFA 6910N, solids: 23 wt %) and 7.6 kgof a 10-wt % acetone solution of VDF. The mixture was gelated, and thesolids were taken out and dried. The solids were subsequently pelletizedin a twin-screw extruder at a die temperature of 160° C. The resultingpellets were melt-extruded at a die temperature of 200° C. with the aidof a single-screw extruder equipped with a slit die at the tip, yieldinga sheet product which was subsequently molded into a sheet with athickness of about 0.05 mm by being passed through a roll press heatedto about 160° C.

Two sheets measuring about 10 cm by 20 cm were prepared, a piece ofglass fabric (E Glass, manufactured by Arisawa Mfg.) with a thickness of0.05 mm was sandwiched between these two sheets, and the components werethen integrated using a press, where a pressure of about 5-10 MPa wasapplied in the thickness direction at a temperature of 220° C.

The integrated sheet was preheated for 1 minutes at 230° C. in athermostat and then kept for 3 minutes at 300° C. The heat-treatedintegrated sheet was subsequently immersed in acetone to dissolve awaythe VDF.

The resulting porous PFA sheet provided with integrated glass fabric wasimpregnated with Treatment Solution 1 by the same method as described inExample 1, then dried with hot air (100° C. or lower), washed withwater, and re-dried. The porous PFA sheet (having the integrated glassfabric and on which titanium dioxide was supported) was subsequentlykept at a temperature of 270-290° C. in an electric oven.

The resulting porous PFA sheet (having the integrated glass fabric andon which titanium dioxide was supported) was an integrated laminate inwhich the upper and lower layers were porous PFA films having a porosityof 70%, and a piece of glass fabric was provided as an intermediatelayer. The mechanical strength thereof, expressed as the longitudinaltensile strength in the planar direction, was 10 kgf per 25 mm of sheetwidth. As in Example 1, titanium dioxide was supported with substantialbonding force on the porous PFA provided with glass fabric.

EXAMPLE 3

A different mixing ratio was used for PFA, and a porous PFA sheet wasfabricated by the same method as in Example 2. The sheet contained apiece of glass fabric and supported titanium dioxide. The PFA porouslayer had a porosity of 60%.

EXAMPLE 4

A porous PFA sheet was impregnated with Treatment Solution 1 as apreliminary step, and was fabricated by the same method as in Example 2.The sheet contained a piece of glass fabric. The PFA porous layer had aporosity of 70%.

Methanol (7200 9) was added to 800 g of a silicon dioxide dispersion(Oscal, manufactured by Shokubai Kasei Kogyo, silicon dioxide 30 wt%-methanol), and 80 g of a surfactant (FC-170C, manufactured by 3M) wasthen added to the resulting solution, yielding Treatment Solution 2. Theabove-described porous PFA sheet provided with glass fabric was immersedin Treatment Solution 2, and the pores in the sheet were impregnatedwith silicon dioxide. The sheet was subsequently dried at 80° C. andheated to 150° C. to deposit the silicon dioxide on the porous PFA sheetprovided with glass fabric.

The porous PFA sheet provided with glass fabric that supported silicondioxide was impregnated with Treatment Solution 1 by the same method asin Example 1, dried with hot air (100° C. or lower), washed with water,and re-dried. The porous PFA sheet provided with glass fabric andimpregnated with titanium dioxide (silicon dioxide was also supported onthis sheet) was then kept at a temperature of 270-290° C. in an electricoven, yielding a porous PFA sheet (porosity: 70%) provided with glassfabric. Silicon dioxide and titanium dioxide were supported on thissheet.

The samples described with reference to Examples 2, 3, and 4 weremeasured for their photocatalytic effect. The measurement methodentailed cutting the aforementioned three types of sheets into 30-cm²segments, immersing the segments in 30 ml of a 50-ppm aqueous solutionof methylene blue, and irradiating the segments with UV light (UV lightintensity: 30 mW/cm²) for 1 hour. As a result, the concentration ofmethylene blue varied as shown in Table 1.

It can be seen in Table 1 that methylene blue was decomposed by thephotocatalytic action of the titanium dioxide supported on the porousPFA sheet provided with glass fabric. It can also be seen that thesample of Example 2 decomposes more of the methylene blue than does thesample of Example 3, and that the effective surface area contributing tothe photocatalytic action of the titanium dioxide is wider in the sampleof Example 2 in proportion to the increased porosity. A comparisonbetween Examples 2 and 4 also demonstrates that a sample that supportssilicon dioxide decomposes a larger amount of methylene blue, indicatingthat the supported silicon dioxide makes it easier for the methyleneblue aqueous solution to penetrate into the material and improves thedecomposition efficiency.

TABLE 1 Results of Measuring Photocatalyzer Carrier DecomposedMethylene-blue Methylene-blue concentration Sample (UV.1 hr irradiation)Example 2 50 ppm → 12 ppm Example 3 50 ppm → 20 ppm Example 4 50 ppm →9.5 ppm

Table 2 shows the results of comparative measurements concerning theamount of moisture passing through the sheets in the samples of Examples2 and 4. The measurement method entailed cutting each of the two typesof sheets into 25-cm² segments and measuring moisture permeability byforcing water through the sheets at a pressure of 0.5 kgf. The resultsin Table 2 indicate that a porous PFA (porosity: 70%) sheet providedwith glass fabric and coated with silicon dioxide and titanium dioxidehas higher moisture permeability (despite the smaller pore diameters ofthe porous PFA layer) than a product coated with titanium dioxide alone,and in actual practice possesses better hydrophilic properties.

TABLE 2 Results of Measuring Water Permeation Rate of PhotocatalyzerCarrier Water Permeation Porous PFA Mean Rate Pore Diameter Samplecc/sec μm Example 2 0.08 1.5 Example 4 1.95 1.26

While particular embodiments of the present invention have beenillustrated and described herein, the present invention should not belimited to such illustrations and descriptions. It should be apparentthat changes and modifications may be incorporated and embodied as partof the present invention within the scope of the following claims.

The invention claimed is:
 1. An article comprising: at least onephotocatalytic substance in the form of particles; and a photocatalystcarrier comprising a hot-melt resin porous article with athree-dimensional reticulated structure, whereby the photocatalyticparticles penetrate into at least one of the surface and the pores ofthe three-dimensional reticulated structure, and are anchored by thehot-melt resin such that the resin encloses only a portion of eachphotocatalytic particle.
 2. The article of claim 1, wherein thephotocatalytic substance comprises titanium dioxide.
 3. The article ofclaim 1, wherein the phtocatalytic substance comprises at least onematerial selected from the group consisting of zinc oxide, iron oxide,cadmium sulfide, cadmium selenide and strontium titanate.
 4. The articleof claim 1, wherein said at least one photocatalytic substance comprisesa combination of two or more photocatalytic substances.
 5. The articleof claim 1, wherein said photocatalyst carrier comprises a hot-meltresin porous article selected from the group consisting oftetrafluoroethylene-hexafluoroethylene copolymers (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),tetrafluoroethylene-hexafluoroethylene-perfluoroalkyl vinyl ethercopolymers (EPA), and tetrafluoroethylene-ethylene copolymers (ETFE). 6.The article of claim 1, wherein said photocatalyst carrier furthercomprises a reticulated layer.
 7. The article of claim 6, wherein saidreticulated layer comprises a glass fabric.
 8. The article of claim 1,wherein said photocatalyst carrier further comprises a hydrophiliccomponent.
 9. An article comprising: at least one photocatalyticsubstance comprising titanium dioxide particles; and a photocatalystcarrier comprising at least one hot-melt resin material selected fromPFA and FEP having a reticulated material comprising glass fabricsandwiched within the photocatalyst carrier, whereby the titaniumdioxide particles penetrate into and are anchored by the hot-melt resinsuch that the resin encloses only a portion of each photocatalyticparticle.
 10. The article of claim 9, further comprising a hydrophiliccomponent comprising silicon dioxide particles coated onto the surfaceof the photocatalyst carrier.
 11. A method for manufacturing aphotocatalyst carrier comprising: applying a photocatalytic substance inthe form of particles onto the surface of a carrier comprising at leastone hot-melt resin porous material; heating the coated carrier to causesaid hot-melt resin to soften, whereby a portion of each photocatalyticparticle penetrates into the resin; and cooling said hot-melt resin sothat said resin contracts around the photocatalytic particles andanchors the photocatalytic particles to the carrier.